You connect them in series (plus to minus) to get higher voltage, you connect them in parallel (plus to plus and minus to minus) to get more current. They don't multiply, they add.

In general, the process of charging a cell from a solar panel is to connect them in parallel with a diode in one line to keep the battery from discharging through the solar panel. The battery uses up the current from the solar panel so much that the solar panel voltage sags down to the battery voltage, then they both rise together. The trick is knowing when to stop.